An OLM file extension is belongs to Outlook 2011 for Mac, It is an email client and Microsoft specially developed this version of Outlook for Mac Operating system. OLM file is a database file of Mac Outlook 2011, Single file can store the whole data of mail client which includes email messages, Contacts, Journals, Tasks, Calendars etc.
Note: The Mac Outlook stores its data in a database file and users can export this database file in OLM file extension. Mac Outlook does not store its data in OLM file from the beginning.
The Maximum size of Mac Outlook OLM file is not defined anywhere but in general the size of OLM file should not be exceed above 2GB because if it exceeds. May be the corruption issue occurs in OLM database file.